docs: def _readASCII(self, filename):
          """ASCII files have no headers. Following synphot, this
          routine will assume the first column is wavelength in Angstroms,
          and the second column is throughput (dimensionless)."""
          self.waveunits = units.Units('angstrom')
          self.throughputunits = 'none'
          wlist, tlist = self._columnsFromASCII(filename)
          self._wavetable = N.array(wlist, dtype=N.float64)
          self._throughputtable = N.array(tlist, dtype=N.float64)